The present invention is directed to a method of filtration and in particular to an improved method of dewatering the filter cake.
Methods and equipment for dewatering have been developed for use in industries ranging from treatment of municipal waste water to filtration of coal extracts and mineral concentrates. The removal of water in a filtration process requires consideration of various factors which relate to such variables as the type of filtration process, the filter, the nature of the particles to be filtered and the target moisture content of the filtered particles. Reduction of the moisture content of a filtered solid may be desirable for many reasons, which include: improved recovery from the filter chamber, reduction in transportation and handling costs, reduction of the energy cost of subsequent thermal drying and increased shelf life.
Filter cake dewatering may be improved by decreasing the viscosity of the retained water by temperature elevation. Thus, the use of steam as a method of lowering the residual moisture of filter cakes is widely known, c.f. U.S. Pat. Nos. 4,310,422, 3,744,543, 3,338,411 and 1,693,417. It is also known to use surfactants and flocculants to reduce moisture in filter cakes. Surfactants, anionic, non-ionic and cationic, are added to the slurry to lower the surface tension and viscosity of the aqueous phase and/or to render the surface of the solid phase more hydrophobic thereby resulting in more effective dewatering. Flocculants improve the sedimentation rate and the filtration rate by trapping fine particles of the precipitate to be filtered. A filter cake prepared with the use of flocculants is more porous and therefore more suitable for air drying. Steams, surfactants and flocculants are complimentary and are often used in combination.
However, conditioning the slurry with fixed concentrations of surfactants or flocculants can escalate processing costs and contaminate the effluent which may then require costly clean-up processing of large amounts of effluent.